Computer aided design (CAD) packages are commonly used to create and edit drawings and other graphic displays on a computer screen or other cathode ray tube (CRT) display. CAD systems are particularly well-suited for producing engineering design drawings by allowing the user to enter a series of commands to produce certain standard elements of the drawing, such as circles, squares and lines, at selected locations on screen to effect a particular design. A locator device such as a pointer, cursor, "mouse" or cross-hair, is used to select the position on the screen at which a particular element is to be drawn. Changes to the drawing can be made directly on screen. The display scale can be changed to zoom in on selected portions of the drawing.